1. Field of the Invention
The present invention relates to a production method of a light wavelength converting element, and specifically to a production method of a light wavelength converting element in which the light wavelength converting element is produced by forming a periodic polarization inversion structure in a unipolarized ferroelectric crystal substrate having a non-linear optical effect.
2. Description of the Related Art
It is widely known that a fundamental wave can be converted to a second harmonic by using a light wavelength converting element in which is provided a region where spontaneous polarization (domain) of a ferroelectric having a non-linear optical effect is periodically inverted. Examples of light wavelength converting elements using this method include an optical waveguide type light wavelength converting element, in which an optical waveguide is formed in a ferroelectric crystal substrate having a non-linear optical effect, a periodic polarization inversion structure, in which orientation of spontaneous polarization of the substrate is inverted, is formed at the optical waveguide, and a fundamental wave guided through the optical waveguide is thereby converted to a second harmonic.
Conventionally, this periodic polarization inversion structure is formed by forming a comb-shaped electrode on a surface of a unipolarized ferroelectric crystal substrate having a non-linear optical effect, at a side where spontaneous polarization of the substrate is negative, forming a plate electrode on a surface thereof, at a side where the spontaneous polarization is positive, and then applying a high voltage between the two electrodes under an insulated condition, such as in a vacuum or in an insulating liquid, such that the comb-shaped electrode has positive potential and the plate electrode has negative potential.
Especially, when the voltage is applied in an insulating fluorine-based liquid, such as Fluorinert (trade name; manufactured by 3M Ltd., United States), having excellent insulating property, a minimum amount of voltage required for inversion (coercive voltage) is several times as much as an amount required if the voltage were applied in a vacuum. For example, if the voltage is applied between a comb-shaped electrode formed with a period of 4.75 xcexcm on an MgOxe2x80x94LiNbO3 substrate and a plate electrode having a suitable distance from the comb-shaped electrode (grounded electrode) so as to invert the polarization, the coercive voltage in a vacuum is about 3 kV, and in Fluorinert is 8 kV. In order to form uniform inversion regions, a voltage of 10 kV needs to be applied for at least 8 seconds.
As described above, the polarization inversion is carried out by applying a voltage which is at the coercive voltage or higher. However, if the applied voltage is too high, cracks are generated near the electrodes, and if a voltage which exceeds a predetermined value (breakdown voltage) is applied, the crystal will be broken. Especially in the above-described optical waveguide type light wavelength converting element, in which the comb-shaped electrode is formed adjacent to the optical waveguide, this generation of cracks is an important problem, because propagation loss of the optical waveguide will be increased by the generation of cracks. Further, if the applied voltage is increased, there is another problem in that a high-voltage power supply and a pressure-tight design are required, which results in a large-scale production apparatus. On the other hand, if the applied voltage is decreased, the generation of cracks is prevented. However, in this case there is a problem in that a region where inversion is not carried out in accordance with the electrode pattern occurs, which reduces production yield of the element.
In Patent Gazette No. 2,969,787, a domain-controlling method for a non-linear ferroelectric optical material, in which a pulse voltage is applied so as to invert a polarization, was suggested. If a DC voltage is applied, too much current flows, and a crystal is thereby broken. On the other hand, in this method, a pulse voltage is applied so that too much current does not flow, and the problem of crystal breakage is thereby avoided.
However, even when the pulse voltage is applied, if the applied voltage is too high, cracks are generated near the electrodes, and if a voltage which is at a breakdown voltage or higher is applied, the crystal is broken as mentioned above. In Patent Gazette No. 2,969,787, there was a description that a voltage is applied in a state in which the non-linear ferroelectric optical material has been heated to a high temperature such that the coercive voltage can be decreased. However, this heating to a high temperature causes problems in that the non-linear ferroelectric optical material is deteriorated and the surface thereof is contaminated. Further, in the wavelength converting element, when a ratio of width of a domain inversion portion of the periodic polarization inversion structure to width of the domain non-inversion portion (duty ratio) is closer to 1:1, wavelength converting efficiency becomes higher. Width and depth of the domain inversion portions of the periodic polarization inversion structure are controlled by the quantity of current flowing when the voltage is applied. However, if the pulse voltage is applied in an insulating liquid, the current flows only in an instant in which the voltage is changed. Accordingly, in this case, it is difficult to control the width and the depth of the domain inversion portion, and this causes a problem in that a desired periodic polarization inversion structure cannot be formed.
In Japanese Patent Application Laid-Open (JP-A) No. 4-335620, there was a description that voltage value and pulse width of the applied pulse voltage are suitably selected such that width and depth of the polarization inversion region can be controlled. In JP-A No. 5-210132, there was a description that pulse width and number of applications of the applied pulse voltage are suitably selected such that width and depth of the polarization inversion region can be controlled. However, if the applied voltage is too high, cracks are generated near the electrodes, and if a voltage which is at a breakdown voltage or higher is applied, the crystal is broken as mentioned above. Further, the above-described problem caused when a pulse voltage is applied in an insulating liquid could not be solved.
In view of the above problems of the prior art, an object of the present invention is to provide a production method of a light wavelength converting element, in which a uniform periodic polarization inversion structure can be formed in a unipolarized ferroelectric crystal substrate having a non-linear optical effect by applying a low voltage thereto, and the light wavelength converting element can be produced with excellent yield.
Further, another object of the present invention is to provide a production method of a light wavelength converting element, in which a uniform periodic polarization inversion structure can be formed even in an insulating liquid, and the light wavelength converting element can be produced with excellent yield.
In order to attain the above objects, a first aspect of the present invention is a method for producing a light wavelength converting element, the method comprising steps of: forming a first electrode having a predetermined pattern on a surface of a unipolarized ferroelectric crystal substrate having a non-linear optical effect, at a side where spontaneous polarization of the substrate is negative, and forming a second electrode opposite to the first electrode; and grounding the second electrode and applying a first pulse voltage such that the first electrode has positive potential, and then grounding the first electrode and applying a second pulse voltage such that the second electrode has negative potential, so as to form a periodic polarization inversion structure in the ferroelectric crystal substrate.
In the present invention, the first electrode having the predetermined pattern is formed on the surface of the unipolarized ferroelectric crystal substrate having a non-linear optical effect, at a side where the spontaneous polarization of the substrate is negative, and the second electrode is formed opposite to the first electrode. When a pulse voltage is to be applied between the first electrode and the second electrode, the second electrode is grounded and the pulse voltage is applied such that the first electrode has positive potential and accumulates charge. Then, the first electrode is grounded and the pulse voltage is applied such that the second electrode has negative potential. Thus, the charge accumulated when the voltage is switched moves such that a current flows. As a result, orientation of polarization is inverted. Thickness of domain inversion portions of the periodic polarization inversion structure is controlled by the quantity of current flowing at the time of inversion. In order to obtain a desired quantity of current, the potential difference needs to be increased. However, in accordance with the method of the present invention, the charge which is accumulated when the second electrode is grounded and the pulse voltage is applied such that the first electrode has positive potential flows all at once, and a sufficient quantity of current can be obtained. Accordingly, even with an applied voltage lower than a conventional voltage, the uniform periodic polarization inversion structure can be formed, and the light wavelength converting element can be produced with excellent yield.
Especially, when the pulse voltage is applied in an insulating liquid, the current flows only in an instant in which the voltage is changed, and thus the quantity of current is insufficient. However, in accordance with the method of the present invention, a sufficient quantity of current can be obtained without increasing the voltage, and the uniform periodic polarization inversion structure can be formed.
The process in which the second electrode is grounded and the pulse voltage is applied such that the first electrode has positive potential and then the first electrode is grounded and the pulse voltage is applied such that the second electrode has negative potential may be repeated a plurality of times. Thus, although the load on the crystal substrate at each time of applying the pulse voltage is reduced, the quantity of current required for the inversion can be obtained. Therefore, the generation of cracks can be more reliably prevented, and charge can be uniformly applied in the crystal substrate. Further, the width and the depth of the domain inversion portion of the periodic polarization inversion structure can be easily controlled.
Further, the pulse voltage may be applied in a state in which the ferroelectric crystal substrate is at a temperature of at least 40xc2x0 C. When the substrate temperature is increased, the coercive voltage is decreased. Thus, if the pulse voltage is applied in the state in which the ferroelectric crystal substrate is at the temperature of at least 40xc2x0 C., the periodic polarization inversion structure can be formed with an even lower applied voltage.
Furthermore, the pulse voltage may be applied in a state in which the ferroelectric crystal substrate is in a vacuum or in an insulating liquid. If the pulse voltage is applied under an insulated condition, current leaks can be prevented, and the light wavelength converting element can be produced with excellent yield. Since the insulating liquid has more excellent insulating property than the vacuum, the pulse voltage is preferably applied in a state in which the ferroelectric crystal substrate is in the insulating liquid.